In IOT (Internet of Thing) and many other location related applications, it is required that an infrastructure be able to estimate the position of one or several wireless objects or target nodes. In a typical location estimation system, there are several reference nodes, or gateways, whose locations are known, and one or several target nodes or motes, whose locations are to be estimated.
In order to estimate the location of a target node, some metrics characterising the positioning information of the target node against the reference nodes, such as the distance or angles, needs to be measured through either radio transmission by the target node and reception by the reference nodes, or the other way around.
The present document will consider mostly, for simplicity, the case where the target node makes radio transmission, which is received by gateways (reference nodes) with the packet's Time of Arrival (TOA) being time-stamped. It must be understood, however, that the invention is applicable also, with appropriate modifications, to the case in which several gateways send simultaneous packets, or packets that are in a determined time sequence, which are received by the target node which, based on the knowledge of their TOA, determine its own position.
Also, the following description describes examples of the invention in which the position of the mobile node is determined in a 2-dimensional plane. Albeit important, this application is not limiting and the invention includes as well variants in which the mobile nodes are localised in a 3-dimensional space.
As known for those skilled in the art, TDOA (Time Difference of Arrival) methods can be used to compute the location if all the gateways are synchronised, but the target node is not. The gateways can be synchronised through either GPS or other synchronisation mechanism. TDOA methods generally involve the determination of the difference between the arrival times of the same packet transmission to two different gateways, which is related, through the known propagation speed, to the difference of the distances from the transmitting node to the two gateways.
Also as known for those skilled in the art, the presence of multipath delay could introduce severe variation and bias on the measurement of TOA and TDOA, which leads to a serious degradation of location accuracy. In a practical environment, either indoor or outdoor, it is most likely that the some or all the gateways may endure a certain level of multipath transmission delay. It is also quite likely that some of the gateways may suffer from heavier multipath delay spread than others. Like other channel-related phenomena, multipath-induced biases on the time of arrival, are difficult to model and estimate.
Localisation estimation system can be generally divided into two categories, i.e.: range-based and range-free methods. TDOA range-based localisation is widely used to achieve good location accuracy. All gateways are assumed to be synchronised, either through GPS or other synchronisation mechanism, however, the target node does not need to be synchronised to gateways. The target node transmits a packet, which is received and time-stamped by gateways to obtain TOA values per each receiving gateway or antenna.
As known for those skilled in the art, at least two TDOA from three different receiving gateways are needed in order to produce a unique estimate of the location of the target node. In some cases, two possible estimates may exist and a further TOA from a fourth gateway may be needed in order to get a final estimate of the target node. When the number of gateways available is more than three, the problem becomes an over-determined one. There are many academic research papers as well as patents addressing different methods to deal with the problem, including Maximum Likelihood algorithm, Least Mean Square Error algorithm, Weighted Least Mean Square Error algorithm, Least Median Error algorithm, and so on. There are also some investigations on the identification of the gateway with multipath delay from other gateways without multipath delay. The system and method presented in this invention falls into the general category of combined “Least Median Error” and “Least Mean Square Error” algorithms with multipath delay gateways identified and eliminated.